The invention relates to a traveling wave amplifier in particular for amplifying a high-frequency signal over a large frequency range.
Traveling wave amplifiers, which are also referred to as distributed amplifiers, serve for amplifying high-frequency signals. In this case, a plurality of amplifier stages are connected up to one another in such a way that their capacitances are isolated but the output currents are additively combined with one another. By virtue of the addition of the amplified output currents, although it is possible initially to obtain only a moderate amplification, the distributed capacitances nevertheless permit a large frequency bandwidth.
So-called traveling field tubes have been used for radiofrequency amplification in the past. Nowadays, semiconductor components (transistors) are used as active, amplifying elements. Increasing miniaturization and the widespread use of microwaves for data transmission in frequency ranges of between 2 and 11 GHz, by way of example, are increasingly leading to the use of CMOS components. Suitable amplifiers are required particularly for future so-called UWB transmission technologies (ultra wide band). In order to provide small radiofrequency amplifiers that are favorable in respect of consumption, the use of normally off CMOS transistors is particularly suitable. However, normally off CMOS transistors require a bias voltage potential or bias potential to be fed in for the purpose of setting their operating points.
FIG. 1 shows a CMOS traveling wave amplifier according to the prior art, such as is illustrated for example in Hee-Tae Ahn, David J. Allstot, “A 0.5-8.5 GHz Fully Differential CMOS Distributed Amplifier” in IEEE Journal of Solid-state Circuits, vol. 37, No. 8, Aug. 2002.
Accordingly, four transistors T1-T4 are provided, which are arranged in parallel in a common-source connection. The drain terminals D of the transistors T1-T4 are connected to a drain line DL, which is connected to a first supply voltage potential VDD, delay elements DE being illustrated schematically in each case between the drain terminals D of the transistors T1-T4. The delay elements DE represent properties of the corresponding lines. The drain line DL is connected, at the drain terminal D of the first transistor T1, to a ground potential GND via a first resistor R1, and it is connected, at the drain terminal D of the fourth transistor T4, to the ground potential GND via a second resistor R2. An output signal OUT can be tapped off at the drain terminal D of the fourth transistor T4. The source terminals of the transistors T1-T4 are connected to the ground potential GND.
The gate terminals G of the transistors T1-T4 are connected to a gate line GL, into which a high-frequency input signal IN is coupled at an input terminal IP. Delay elements DE, as line properties, are depicted schematically between the nodes of the gate line GL to which the gate terminals G of the transistors T1-T4 are connected. The gate line GL is connected, at the gate terminal G of the fourth transistor T4, to the ground potential GND via a third resistor R3. Furthermore, a bias voltage source VS is connected between the gate line GL and the ground potential GND, and raises the gate line GL and thus the gate terminals G of the transistors T1-T4 to a bias potential UBIAS. This last is necessary since the transistors T1-T4 are provided as normally off CMOS transistors. However, the coupling-in of the bias potential via the input or the gate line is very sensitive to temperature and manufacturing fluctuations. Moreover, the bias potential UBIAS has to be carefully tracked in the event of changes in the input signal level in order to ensure the operating point setting.
Therefore, it is an object of the present invention to provide a traveling wave amplifier which has normally off MOS transistors, the operating point setting thereof being performed favorably in respect of complexity in a manner tolerant toward temperature and manufacturing fluctuations. In particular, the intention is for the traveling wave amplifier to be able to be embodied in differential fashion and to have a low current consumption.